void VP8SetSegmentParams(VP8Encoder* const enc, float quality) {
int i;
int dq_uv_ac, dq_uv_dc;
const int num_segments = enc->config_->segments;
const double amp = SNS_TO_DQ * enc->config_->sns_strength / 100. / 128.;
const double c_base = QualityToCompression(quality);
for (i = 0; i < num_segments; ++i) {
// The file size roughly scales as pow(quantizer, 3.). Actually, the
// exponent is somewhere between 2.8 and 3.2, but we're mostly interested
// in the mid-quant range. So we scale the compressibility inversely to
// this power-law: quant ~= compression ^ 1/3. This law holds well for
// low quant. Finer modelling for high-quant would make use of kAcTable[]
// more explicitely.
// Additionally, we modulate the base exponent 1/3 to accommodate for the
// quantization susceptibility and allow denser segments to be quantized
// more.
const double expn = (1. - amp * enc->dqm_[i].alpha_) / 3.;
const double c = pow(c_base, expn);
const int q = (int)(127. * (1. - c));
assert(expn > 0.);
enc->dqm_[i].quant_ = clip(q, 0, 127);
}
// purely indicative in the bitstream (except for the 1-segment case)
enc->base_quant_ = enc->dqm_[0].quant_;
// fill-in values for the unused segments (required by the syntax)
for (i = num_segments; i < NUM_MB_SEGMENTS; ++i) {
enc->dqm_[i].quant_ = enc->base_quant_;
}
// uv_alpha_ is normally spread around ~60. The useful range is
// typically ~30 (quite bad) to ~100 (ok to decimate UV more).
// We map it to the safe maximal range of MAX/MIN_DQ_UV for dq_uv.
dq_uv_ac = (enc->uv_alpha_ - MID_ALPHA) * (MAX_DQ_UV - MIN_DQ_UV)
/ (MAX_ALPHA - MIN_ALPHA);
// we rescale by the user-defined strength of adaptation
dq_uv_ac = dq_uv_ac * enc->config_->sns_strength / 100;
// and make it safe.
dq_uv_ac = clip(dq_uv_ac, MIN_DQ_UV, MAX_DQ_UV);
// We also boost the dc-uv-quant a little, based on sns-strength, since
// U/V channels are quite more reactive to high quants (flat DC-blocks
// tend to appear, and are displeasant).
dq_uv_dc = -4 * enc->config_->sns_strength / 100;
dq_uv_dc = clip(dq_uv_dc, -15, 15); // 4bit-signed max allowed
enc->dq_y1_dc_ = 0; // TODO(skal): dq-lum
enc->dq_y2_dc_ = 0;
enc->dq_y2_ac_ = 0;
enc->dq_uv_dc_ = dq_uv_dc;
enc->dq_uv_ac_ = dq_uv_ac;
SetupMatrices(enc);
SetupFilterStrength(enc); // initialize segments' filtering, eventually
}
void VP8SetSegmentParams(VP8Encoder* const enc, float quality) {
int i;
int dq_uv_ac, dq_uv_dc;
const int num_segments = enc->segment_hdr_.num_segments_;
const double amp = SNS_TO_DQ * enc->config_->sns_strength / 100. / 128.;
const double Q = quality / 100.;
const double c_base = enc->config_->emulate_jpeg_size ?
QualityToJPEGCompression(Q, enc->alpha_ / 255.) :
QualityToCompression(Q);
for (i = 0; i < num_segments; ++i) {
// We modulate the base coefficient to accommodate for the quantization
// susceptibility and allow denser segments to be quantized more.
const double expn = 1. - amp * enc->dqm_[i].alpha_;
const double c = pow(c_base, expn);
const int q = (int)(127. * (1. - c));
assert(expn > 0.);
enc->dqm_[i].quant_ = clip(q, 0, 127);
}
// purely indicative in the bitstream (except for the 1-segment case)
enc->base_quant_ = enc->dqm_[0].quant_;
// fill-in values for the unused segments (required by the syntax)
for (i = num_segments; i < NUM_MB_SEGMENTS; ++i) {
enc->dqm_[i].quant_ = enc->base_quant_;
}
// uv_alpha_ is normally spread around ~60. The useful range is
// typically ~30 (quite bad) to ~100 (ok to decimate UV more).
// We map it to the safe maximal range of MAX/MIN_DQ_UV for dq_uv.
dq_uv_ac = (enc->uv_alpha_ - MID_ALPHA) * (MAX_DQ_UV - MIN_DQ_UV)
/ (MAX_ALPHA - MIN_ALPHA);
// we rescale by the user-defined strength of adaptation
dq_uv_ac = dq_uv_ac * enc->config_->sns_strength / 100;
// and make it safe.
dq_uv_ac = clip(dq_uv_ac, MIN_DQ_UV, MAX_DQ_UV);
// We also boost the dc-uv-quant a little, based on sns-strength, since
// U/V channels are quite more reactive to high quants (flat DC-blocks
// tend to appear, and are displeasant).
dq_uv_dc = -4 * enc->config_->sns_strength / 100;
dq_uv_dc = clip(dq_uv_dc, -15, 15); // 4bit-signed max allowed
enc->dq_y1_dc_ = 0; // TODO(skal): dq-lum
enc->dq_y2_dc_ = 0;
enc->dq_y2_ac_ = 0;
enc->dq_uv_dc_ = dq_uv_dc;
enc->dq_uv_ac_ = dq_uv_ac;
SetupFilterStrength(enc); // initialize segments' filtering, eventually
if (num_segments > 1) SimplifySegments(enc);
SetupMatrices(enc); // finalize quantization matrices
}